First, identify which disks you are using for scratch (in Photoshop's preferences).
Then make sure they have plenty of space.
When you make a new document, double check that you are creating it at the size you think you are (and not mixing up pixels and inches, or at 9000 dpi or something).

Startup disk has 45 GB free.Two external drives attached, each with more than 1 TB free.

I've tried redoing the composite. Restarting my mac. Reopening the image, etc. All lead to scratch disk full. But if I flip to finder, I see 28 GB free on startup disk, implying that PS is using 17 GB but believes it's using more than 45 GB.